With the large number of power amplifiers used in todays car radios and boosters, there is a growing need to control the amplifier by the microcontroller that is usually available in such large systems.The microcontroller should be able to control the mute and standby functions of the amplifier, and should be able to read out diagnostic information. This information could concern short circuited outputs, high chip temperature, or distortion due to output voltage clipping.The TDA1551 is a dual 22 W or quad 6 W audio power amplifier that has all these features. It has an on-chip I2C bus interface.This paper deals with the I2C bus protocol only briefly. Special attention is paid to the interfaces between the analog and digital circuits.The amplifiers may be used independently. The fact that they share the same encapsulation requires some attention with the design of the external heat sink, especially when the amplifiers are unevenly loaded.